docs: def subcorpus(self, selector):
          """
          Generates a new :class:`.Corpus` using the criteria in ``selector``.
  
          Accepts selector arguments just like  :meth:`.Corpus.select`\.
  
          .. code-block:: python
  
             >>> corpus = Corpus(papers)
             >>> subcorpus = corpus.subcorpus(('date', 1995))
             >>> subcorpus
             <tethne.classes.corpus.Corpus object at 0x10278ea10>
  
          """
          subcorpus = self.__class__(self[selector],
                             index_by=self.index_by,
                             index_fields=self.indices.keys(),
                             index_features=self.features.keys())
  
          return subcorpus